1. Field of the Invention
The present invention relates to an objective lens to be provided at a tip of an endoscope, and more particularly to an objective lens for an endoscope (hereinafter, sometimes referred to as “endoscopic objective lens”) capable of performing focal adjustment (hereinafter, sometimes referred to as “focusing”), by moving part of the system lenses, in states from a state suited for observing an object in its entirety (hereinafter, referred to as “distant-side observation state) to a state suited for observing, with magnification, a part of the object (hereinafter, referred to as “a nearside magnifying observation state”).
2. Description of Related Art
As for the endoscopic objective lenses of this kind, there are known those described in Japanese Patent No. 2876252 and JP-A-2001-91832.
The endoscopic objective lens, described in Japanese Patent No. 2876252, is arranged with four groups whose refractive powers are negative, positive, negative and positive in order from the object side. By moving the third group along the optical axis, focal adjustment can be performed from a distant-side observation state to a nearside magnifying observation state. By virtue of the lens groups movable in position, observation is available not only at the both ends for the most-distant and nearest points but also at the intermediate region between the both ends.
Meanwhile, the endoscopic objective lens, described in JP-A-2001-91832, is arranged with four groups whose refractive powers are negative, positive, negative and positive in order from the object side. By moving the second and third groups or the third and fourth groups along the optical axis, focal adjustment can be performed from a distant-side observation state to a nearside magnifying observation state. Furthermore, the magnification of the lens in use can be desirably changed in the intermediate region between the both ends for the most-distant and nearest points.
However, in the endoscopic objective lenses described in Japanese Patent No. 2876252 and JP-A-2001-91832, the observing magnification greatly changes upon focusing in the nearside magnifying observation state. Thus, the subject to be observed (i.e., the object) is ready to be placed out of the field of view. For the endoscopic objective lens, it is considerably difficult to perform focal adjustment where the observing magnification changes greatly, because the depth of observation (the depth of field) is narrow in the nearside magnifying observation state.